1. Field of the Invention
This invention generally relates to gas-liquid contactors used in the removal of acidic gases, such as from utility and industrial flue gases. More particularly, this invention is directed to a wet flue gas desulfurization (FGD) process and apparatus that remove nitrogen oxides (NOx) from a flue gas following the removal of acidic gases such as sulfur dioxide.
2. Description of the Related Art
Acidic gases, including sulfur dioxide (SO2), hydrogen chloride (HCl) and hydrogen fluoride (HF), are known to be hazardous to the environment, and as a result their emission into the atmosphere is closely regulated by clean air statutes. For the removal of acidic gases from flue gases produced by utility and industrial plants, gas-liquid contactors and absorbers, or scrubbers, are widely employed. Scrubbers generally employ a liquid media that is brought into intimate contact with a flue gas to remove acidic gases by absorption. The process by which acidic gases are removed from flue gases in this manner is generally referred to as wet flue gas desulfurization (wet FGD).
The cleansing action produced by scrubbers is generally derived from the passage of a flue gas through a tower cocurrently or countercurrently to a descending liquid medium. Calcium-based slurries, sodium-based solutions and ammonia-based solutions are typical alkaline scrubbing media used in flue gas scrubbing operations. The cleansed gases are allowed to exit the tower, typically passing through a mist eliminator to atmosphere. The liquid medium and its absorbed gases are collected in a tank, typically at the bottom of the tower, where the absorbed gases are reacted to form byproducts that are useful or at least not harmful to the environment. While scrubbers utilizing calcium-based slurries generally perform satisfactorily, their operation results in the production of large quantities of wastes or gypsum, the latter having only nominal commercial value. In contrast, ammonia-based scrubbing processes have been used in the art to produce a more valuable ammonium sulfate fertilizer, as taught by U.S. Pat. Nos. 4,690,807 and 5,362,458, each of which are assigned to the assignee of the present invention. In these processes, the scrubbing solution is accumulated in a tank where the absorbed sulfur dioxide reacts with ammonia (NH3) to form ammonium sulfite ((NH4)2SO3) and ammonium bisulfite (NH4HSO3), which are oxidized in the presence of sufficient oxygen to form ammonium sulfate ((NH4)2SO4) and ammonium bisulfate (NH4HSO4), the latter of which reacts with ammonia to form additional ammonium sulfate. A portion of the ammonium sulfate solution and/or ammonium sulfate crystals that form in the solution can then be drawn off to yield the desired byproduct of this reaction.
Nitrogen oxides (NOx), which include nitric oxide (NO) and nitrogen dioxide (NO2), are also commonly found in flue gases produced by utility and industrial plants. The presence of NOx in the effluent of a scrubber is environmentally undesirable because of links to smog and ozone deterioration. Because nitric oxide readily oxidizes at room temperature (about 20° C. to about 25° C.) to form nitrogen dioxide, nitric oxide present in flue gases tends to form nitrogen dioxide which, depending on atmospheric conditions, can produce an unsightly yellow-brown plume. The elimination of NOx from flue gases has typically focused on the avoidance of forming nitrogen dioxide. For example, nitrogen gas (N2) is produced in processes involving NOx dissociation by selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR). As the primary component of air, the release of nitrogen gas into the atmosphere does not pose an environmental concern. However, an ongoing demand of desulfurization processes is the reduction of emissions. From an economics standpoint, it is also desirable for a desulfurization process to produce valuable byproducts. Electron beam and low NOx burners have also been proposed as methods for removing Nox. For example, irradiation with an electron beam has been used to convert NOx to ammonium nitrate (NH4NO3) in the presence of ammonia, as disclosed in U.S. Pat. Nos. 5,834,722 and 6,179,968. However, disadvantages of ion beam treatments include their relatively high cost and low efficiency.